Generally, Styrofoam or sponge is known as a representative example of low-density material. The materials each have closed cell and open cell microstructures in a density that is lower than one tenth of the water. The Styrofoam or sponge has been widely used in people's daily lives for many applications including insulation and packaging for more than half a century.
Recently, growing attentions have been placed on a lighter material such as ultra-low dense material including aerogel that is one hundredth the density of water. The aerogel is a porous material consisting of micro-sized cells, which is formed after removing liquid from a gel-type material. FIG. 1 is a photograph of silica aerogel which is called “frozen smoke” for semi-transparent property thereof (T. M. Tillotson, L. W. Hrubesh, J. Non-Cryst. Solids 145, 44, 1992). Meanwhile, in addition to silica, the aerogel can be made from organic material such as metal oxides or polymer, and thanks to ultra-lightweightness, high insulating characteristic and specific surface area thereof, it has found a wide variety of applications including window insulating materials, building materials, tennis rackets, oil-absorbing sponges, etc.
FIG. 2 is structure photograph of the multi-walled carbon nano tube (MWCNT) which is a new aerogel that was introduced early in 2011. MWCNT consists of carbon nano tube (CNT) which is known for superb strength, and has hierarchical structure of micrometer-size cells and nanometer-sized multilayer walls, in a very low density of 4 mg/cm3 (J. Zou et al., ACS Nano 4, 7293, 2010). When filled with resin, MWCNT can be extended to a size that is several thousand times larger than the original size. Accordingly, MWCNT can make highly elastic energy storage, and electric conductivity thereof allows its applications in a variety of fields including high performance sensor or displays, to name just a few.
FIG. 3 illustrates an example of metal foam which is another example of low-density material. In the case of the recently-developed metal foam consisting of nano-sized cells, its density is known to be 8 mg/cm3 (B. C. Tappan et al., J. Am. Chem. Soc. 128, 6589, 2006). Despite heavier weight than aerogel, the metal foam is considered to be the prominent candidate for the electrode material due to conductivity of the metal and high surface area. The metal foam can be made from a variety of metals including iron, cobalt, copper, silver, etc.
Meanwhile, the low-density materials such as Styrofoam, sponge, aerogel or metal foam have degrading strength and stiffness due to defects, i.e., due to the presence of irregular cells therein.
Recently, the Science magazine (November, 2011) introduced a new concept of ultra-low density metal micro-lattice which has 1/1000 density of the water and which has three-dimensional lattice truss structure (T. A Schaedler, et al., Science, Vol. 334. Pp. 962-965 Nov. 18, 2011). A mask having a regular pattern of micro-holes bored in a liquid photo monomer bulk which solidifies upon exposure to ultraviolet ray, is disposed on a specific surface of the bulk, and then ultraviolet rays are emitted. Upon exposure to a plurality of ultraviolet beams passed through the mask, the liquid photosensitive resin is solidified. The solid photosensitive resin has a higher density than the liquid photosensitive resin, and thus has “self propagating” phenomenon, which means that total reflection of the ultraviolent beam is induced inside the solid photosensitive resin so that the beams are moved straightforwardly without being dispersed. By varying the direction of radiating ultraviolet beams and repeating the above-described process in a plurality of directions, the lattice consisting of solid photosensitive resin is formed inside the liquid photosensitive resin bulk. After removing the liquid photosensitive resin from the solid photosensitive resin lattice, nickel alloy (NiP) is plated on the surface of the solid photosensitive resin lattice by autocatalytic electroless plating. Then the solid photosensitive resin is removed by chemical etching, leaving the micro-lattice as illustrated in FIG. 5. The outer shape of the completed micro-lattice is polyhedron on a several mm scale, but has several hundreds of micro-meter truss elements each having a hollow tubular shape having 100 nano-meter scale wall thickness. The geometric structure of the micro-lattice is identical to the solid photosensitive resin structure as used, and in an aspect of molding, it can be understood to be the engrave manner. The multi-scale hierarchical structure such as micro-lattice is known as the method for achieving lightweightness and high strength (T. Bhat, T. G. Wang, L. J. Gibson, SAMPE J Vol. 25 (1989) pp. 43, R. Lakes. Nature Vol. 361 (1993) pp. 511.).
The micro-lattice is based on photo-lithography which is well-established type of semiconductor manufacturing process and which provides advantages such as ultra-low density, and relatively higher strength and stiffness. However, such a small diameter size of the mask pattern limits penetration of thin-diameter ultraviolet beams to several mm maximum, thus hindering fabrication of the structure on a large scale. Further, because the ultraviolet beams emitted only in out-of-plane direction result in truss elements, the lack of truss elements in an in-plane direction can deteriorate structural stability. FIG. 6 illustrates the collapsing under compression loads exerted on the unit cells of the micro-lattice. The absence of in-plane truss elements at a dotted-location in unit cells causes inability to resist horizontal deformation of the truss elements, and as a result, bending occurs easily.
As a separate way of making three-dimensional multi-porous structure based on lithography, four of the present inventors including Ki-Ju KANG proposed a method of processing a plurality of parallel, hexagonal pillar-form pores in the solid bulk, as disclosed in Korean Patent Registration No. 0794358. Specifically, among total six directions including three in-plane directions having 60 or 120 degree azimuth, and three out-of-plane directions, the method processes hexagonal pillar form of pores in three to six directions. FIG. 7 illustrates the shapes in each stage of processing hexagonal pillar form of pores in one to four directions. The final resultant form of fabricated structure is similar to the three-dimensional Kagome truss (S. Hyun, A. M. Karlsson, S. Torquato, A. G. Evans, 2003. Int. J. of Solids and Structures, Vol. 40, pp. 6989-6998). The method for processing hexagonal pillar form of pores includes not only the macro processing technique such as electric discharge processing, high energy particle processing, or laser processing, but also ultrafine processing technique such as high aspect ratio (HAR) MEMS which can process micrometer-scale pores. FIG. 9 schematically illustrates processing pores with mask used in high energy particle processing and entrance of two-directional high energy beams. However, the high energy particle processing takes considerable amount of cost, while it is applicable in extremely limited field of technology. The rest methods are also quite disadvantageous, in cost and practical aspects, for processing elongated, hexagonal pillar form of pores with micro-apertures. Further, when fabricated by these methods, the resultant three-dimensional porous structures reach only 50% to 70% of porosity.